1. Field of the Invention
The present invention relates to a solid electrolyte with high ion-conductivity of protons (hydrogen ions), hydroxide ions, and the like which is cheap and exhibits high conductivity even in an alkaline form, and also can stably keep high conductivity because of a small amount of the leak of a compound bearing conductivity even in a wet state, a method for manufacturing the same, and an electrochemical system using the solid electrolyte, such as a fuel cell.
2. Description of the Related Art
Conventionally, electrolytic devices such as fuel cells, dehumidifiers, and electrolytic hydrogen-producing devices have been practically used as electrochemical systems employing a proton-conducting solid electrolyte. In particular, the applications of proton-conducting solid electrolytes which operate at room temperature are wide-ranging. For example, in a solid polymer fuel cell, current flows and electric energy is obtained by an electrochemical oxidative reaction of hydrogen supplied to a negative electrode shown by the following formula (1), an electrochemical reductive reaction of oxygen supplied to a positive electrode shown by formula (2), and a reaction based on proton transfer in the electrolyte between the positive electrode and the negative electrode.H2→2H++2e−  (1)1/2O2+2H++2e−→H2O  (2)
Although there are direct methanol-type fuel cells in which methanol is the fuel supplied to the negative electrode and fuel cells using substances other than hydrogen or methanol as the fuel supplied to the negative electrode, in these cases also, the fuels are electrochemically oxidized at the negative electrode to release protons in a similar manner. Thus, it is possible to operate by using the proton-conductive solid electrolyte.
Electrolytic hydrogen-producing devices, for example, are practically used as electrolytic devices. Electrolytic hydrogen-producing devices produce hydrogen on the basis of a reaction opposite to the above-mentioned formulae (1) and (2) in a fuel cell and have the advantage that a hydrogen gas is unnecessary since it is possible to obtain high-purity hydrogen on-site by using only water and electric power. Also, by using a solid electrolyte, it is possible to easily carry out electrolysis only by the introduction of pure water including no electrolyte. In the paper industry, the on-site manufacture of hydrogen peroxide for bleach by a similar system has been attempted by an electrolytic method using the following formula (3) (refer to Electrochemistry, 69, No. 3, 154 to 159 (2001)).O2+H2O+2e−→HO2−+OH−  (3)
Dehumidifiers have a structure in which the proton-conducting solid electrolyte film is sandwiched between the positive electrode and the negative electrode, similar to fuel cells or the hydrogen-producing devices. When a voltage is applied between the positive electrode and the negative electrode, water is split into protons and oxygen at the positive electrode by the reaction in the following formula (4). The protons, which have moved through the solid electrolyte to the negative electrode, bind with oxygen in the air to form water again by the reaction of formula (5). As a result of these reactions, dehumidification is carried out at the positive electrode by water transfer from the positive electrode to the negative electrode.H2O→1/2O2+2H++2e−  (4)1/2O2+2H++2e−→H2O  (5)
It is also possible to split water and to dehumidify by an operation principle similar to electrolytic hydrogen-producing devices. Also, an air conditioner combined with a moisture evaporation cold air device has been proposed (refer to Collected papers of the 2002 National Meeting of the Institute of Electrical Engineers, P3373 (2000)).
Various kinds of sensors, electrochromic devices, and the like are systems based on an operation principle essentially similar to that mentioned above. It is possible to use a proton-conducting solid electrolyte since these systems operate by the transfer of protons through the electrolyte between two kinds of different redox pairs of positive and negative electrodes. Presently, an experimental study with respect to these systems using proton-conducting solid electrolytes is being carried out.
For hydrogen sensors, for example, the variation of electrode potential dependent on the hydrogen concentration when hydrogen is introduced in the reactions of the above-mentioned formulae (4) and (5) can be used. Furthermore, using the variation of electrode potential or the variation of ion conductivity, it is also possible to apply to a humidity sensor.
When a substance such as WO3 is employed as the negative electrode and an electric field is applied to it, the electrochromic device makes a color on the basis of the reaction of the following formula (6) and can be used in displaying devices and lightproof glass. This system is also operated by donating and accepting protons for the negative electrode, and it is possible to use the proton-conductive solid electrolyte.WO3+xH++xe−→HxWO3 (Coloring)  (6)
Primary batteries, secondary batteries, optical switches, and electrolyzed water-manufacturing devices can be given as examples of other electrochemical systems which are considered to operate by using a proton-conducting solid electrolyte according to their mechanism. For nickel hydride batteries, as an example of the secondary batteries, a hydrogen-absorbing alloy is used as the negative electrode, a nickel hydroxide is used as the positive electrode, and alkaline electrolytic solution is used as the electrolytic solution. As shown by the following formulae (7) and (8), at charging and discharging, electrochemical reduction and oxidation of the proton occurs at the negative electrode, and hydrogen is stored in the hydrogen-absorbing alloy.(Charging) H2O+e−→H (Absorbing)+OH−  (7)(Discharging) H (Absorbing)+OH−→H2O+e−  (8)
As shown by the following formulae (9) and (10), the electrochemical oxidation and reduction of the nickel hydroxide occurs.(Charging) Ni(OH)2+OH−→NiOOH+H2O+e−  (9)(Discharging) NiOOH+H2O+e−→Ni(OH)2+OH−  (10)
The charging and discharging reactions of this battery are conducted by the proton or the hydroxide ion moving in the electrolyte. Although it is possible to use the proton-conducting solid electrolyte according to its mechanism, alkaline electrolytic solution, which is not a solid electrolyte, is usually conventionally used.
An optical switch using yttrium as the negative electrode has been proposed (refer to J. Electrochem. Soc., Vol. 143, No. 10, 3348 to 3353 (1996)). When an electric field is applied thereto, the yttrium is hydrogenated as shown in the formula (11) to allow light to pass therethrough. As a result, it is possible to switch between transmission and nontransmission of light by the electric field. Although it is possible to use the proton-conductive solid electrolyte in this system, alkaline electrolytic solution is used in the prior art.Y+3/2H2O+3e−→YH3+3OH−  (11)
Electrolyzed water is water which is produced by an electrolysis reaction. Although efficacy is depending on the reduction side or the oxidation side, the electrolyzed water has a healthful effect, a bactericidal effect, a detergent effect, and an effect of promoting the growth of farm products. It is possible to use as drinking water, water for food preparation, cleaning water, agricultural water, and the like. Although the electrolysis reaction is promoted when water includes an electrolyte, however, in some cases, the electrolyte as a solute in water is needed to be removed. When a solid electrolyte is used as the electrolyte, it is unnecessary to remove the solid electrolyte from the water.
In many of the above-mentioned electrochemical systems such as fuel cells, electrolytic devices, and dehumidifiers, which have already been put to practical use, a perfluorosulfonic acid membrane sold under the tradename of Nafion by DuPont is employed as a solid electrolyte. Also, the applicant of the present application has already provided solid electrolytes comprising an inorganic/organic hybrid compound of a zirconic acid compound and polyvinyl alcohol (refer to Japanese Unexamined Patent Publication (Kokai) No. 2003-242832; and Japanese Unexamined Patent Publication (Kokai) No. 2004-146208). Furthermore, for these solid electrolytes, a casting method, which is the method for forming membrane by casting an aqueous raw material solution on a flat plate and removing the water of the solvent by heating, is applied. (refer to Japanese Unexamined Patent Publication (Kokai) No. 2004-285458). These hybrid compounds can be prepared by neutralizing a zirconium salt or an oxyzirconium salt with alkali in the situation in which polyvinyl alcohol coexists and exhibit comparatively high proton (hydroxide ion) conductivity by impregnating with alkali such as sodium hydroxide, sodium silicate, or sodium carbonate.
The applicant of the present application provides means for more satisfactorily manufacturing these solid electrolytes, through the steps of heating a solution in which a solvent including water, polyvinyl alcohol, and a zirconium salt or an oxyzirconium salt coexist at 50° C. or higher and pH of 7 or less to hydrolyze the zirconium salt or the oxyzirconium salt; removing the solvent; and contacting the solution with alkali (refer to Japanese Patent Application No. 2007-84374).
On the other hand, an anion-exchange membrane functions as a hydroxide ion conductive material and has conventionally been used. For example, an anion-exchange membrane as an electrolyte for a fuel cell (refer to Japanese Unexamined Patent Publication (Kokai) No. 2000-331693) and an anion-exchange membrane containing an inorganic filler added therein are provided (refer to Japanese Unexamined Patent Publication (Kokai) No. 2004-217921). As a polymer electrolyte, composition comprising a nitrogen-containing organic compound and a metal hydroxide (refer to Japanese Unexamined Patent Publication (Kokai) No. 2002-525803), or a conventional polymer solid electrolyte film comprising inorganic fine particles, an electrolyte salt and a polymer, which does not necessarily have hydroxide ion conductivity, is disclosed (refer to Japanese Unexamined Patent Publication (Kokai) No. 2004-339422).